The present invention is a diesel fuel composition for an increased cetane number vis-a-vis conventional diesel fuel compositions and generally comprises one or more compounds selected from the dialkoxy alkane (DAAK) chemical family.
A diesel fuel is a broad class of petroleum products which includes distillate or residual materials (or blends of these two) from the refining of crude oil and which is used in compression ignition or diesel engines. The two primary criteria used to define diesel fuel are distillation range (generally between 150.degree. and 380.degree. C. or 302.degree. and 716.degree. F.) and specific gravity range (between 0.760 and 0.935 at 59.degree. F. or 15.degree. C.). The properties of diesel fuel greatly overlap those of kerosene, jet fuels, and burner fuel oils and thus all these products are generally referred to as intermediate distillates.
The cetane number of a diesel fuel is roughly analogous to the octane number of gasoline. A high cetane number indicates the ability of a diesel engine fuel to ignite quickly after being injected into the combustion cylinder.
Prior to reviewing the prior art with regard to diesel fuel compositions comprising DAAKs, it is worth noting some background information on the DAAK chemical family, including alternative nomenclature. DAAKs can be represented as R--O--X--O--R where R=C.sub.n H.sub.2n+1, O=oxygen and X=C.sub.m H.sub.2m. Probably the best known compound in this family is dimethoxy methane (DMMT) where n and m in the above formula are equal to 1 and which is more commonly referred to as methylal. Other compounds in this family which are the subject matter of the present invention include dimethoxy ethane (DMET) where n is again equal to 1 but m is equal to 2, and dimethoxy propane (DMPP) where n is again equal to 1 but m is equal to 3. Other common nomenclature for DAAKs is alkylene glycol dialkyl ethers. Similarly, other common nomenclature for DMMT, DMET and DMPP is, respectively, methylene glycol dimethyl ether, ethylene glycol dimethyl ether and (as it relates to 1,2 DMPP) propylene glycol dimethyl ether.
DMMT is taught as a cetane improving additive for diesel fuel. Specifically, a study by Southwest Research Institute for the US Department of Energy (as reported in OSTI as DE94006949, June 1994) teaches that DMMT (referred to as methylal in this study) may have possible use as a diesel fuel additive/replacement because it reduces smoke emissions and because it has a favorable cetane number. A repeat test by Southwest Research Institute on the Applicant's behalf, however, indicates that DMMT has a cetane number of only 29 (as compared to a cetane number of approximately 40 for conventional diesel fuel) and is not a cetane improver when added to diesel fuel.
DMET is taught as a diesel fuel additive in small (less than 5 weight %) concentrations for the purpose of soot and smoke suppression. See for example U.S. Pat. No. 3,594,136, U.S. Pat. No. 3,594,140, U.S. Pat. No. 3,615,292 and GB Patent Specification 1,246,853. DMET was also studied as a possible soot reducing diesel fuel replacement by Beatrice et al. in a 1996 article in IMech E (C517/023/96) where it was noted that DMET has a cetane number of 98. One study by Southwest Research Institute as reported in the SAE Technical Paper Series (950250) also teaches DMET (which was referred to as ethylene glycol dimethyl ether and "monoglyme" in this study) as a diesel fuel additive at moderate concentrations, specifically at concentrations of 5.62 mass % (5.5 volume %) and 11.24 mass % (11.1 volume %). The purpose of adding DMET to diesel duel in this study was not for cetane improvement, however, but for the purpose of increasing the oxygen level of the diesel fuel so that the effect of oxygen level on emission levels could be determined. Although this study also adjusted the cetane number of the diesel fuel so that the effect of cetane number on emission levels could be similarly determined, the cetane improver additive was 2-ethylhexyl nitrate and not DMET. Any cetane improvement attributable to DMET in this study was inadvertent. This study did note that oxygenating the diesel fuel generally increased cetane number in proportion to the amount of DMET added. Applicant's testing, however, indicates that although DMET has a very high cetane number of 105, it is not a significant cetane improver when added to diesel fuel except at concentrations above approximately 25 volume %.
DMPP, and other DAAKs besides DMMT and DMET, are not taught as diesel fuels or additives thereto.